Plato’s Republic introduces and employs an elaborate psychological theory whose core commitment it is that human motivation comes in three forms: rational, spirited, and appetitive. The Brute Within offers a detailed philosophical analysis of appetitive motivation and of Plato’s conception of appetite as a part of the soul. In doing so, it explores both the psychological theory of the Republic and its afterlife in Plato’s later dialogues as well as in Aristotle’s psychology and ethics. It shows that Plato’s Timaeus, a relatively late dialogue, preserves the substance of the Republic’s conception of appetite as a distinct part of the soul. At the same time, the Timaeus offers a number of important clarifications and amplifications of the theory of the tripartite soul, whose full significance emerges once the Timaeus is read in the context of a number of other later dialogues, most importantly the Theaetetus and the Philebus. In turning to Aristotle’s psychological theory and moral psychology, the book calls attention to the remarkable continuity between Aristotle’s and Plato’s thought in this area. It shows how Aristotle made Plato’s psychological theory his own both by modifying it where appropriate and by giving it a more determinate and precise formulation.

The paper argues that Aristotle's mature theory of perception in the De Anima assigns prominent explanatory roles both to a technical notion of assimilation of sense to sense-object, according to which such assimilation is identical with perceiving, or with perceiving considered in a certain way, and to assimilation of a more ordinary kind, in which one thing (such as the sense-organ in question) comes to be like another by taking on one of its features in a straightforward way. If this is along the right lines, neither Burnyeat's spiritualism nor Sorabji's (or Everson's) literalism does justice to Aristotle's conception of the likeness between perceiver and sense-object in the act of perception.